Such a pressure filter is known from EP 0 479 077 A1. This pressure filter works with a continuous cleaning of the filter surface and is especially destined for the separation of solids from suspensions. A plurality of pressure-proof filter elements separating the fluid compartment from the filtrate compartment and forming the filter surface and the backwashing means connected to the reject drain and having a backwashing head sealing a part of the filter surface with respect to the fluid compartment belong to the fundamental construction of such a pressure filter. The backwashing head and the filter surface are movable with respect to one another. The fluid to be filtered is supplied to the pressure filter with a pressure p.sub.1 and pressurizes the filter elements with this pressure p.sub.1. The filtrate is drained from the filtrate compartment with a lower pressure p.sub.2. In the reject drain and the backwashing compartment restricted by the backwashing head a pressure p.sub.3 is adjustable which is always smaller than the pressure p.sub.2 within the filtrate compartment. A filtration is carried out with the pressure difference between the pressure p.sub.1 of the fluid compartment and the pressure p.sub.2 of the filtrate compartment, wherein solids are deposited at the filter surface and form a cover layer. This cover layer is continuously cleaned, namely by backwashing, i.e. a conversion of the flow direction through the filter elements. The backwashing occurs at that part of the filter surface which is sealed by the backwashing head with regard to the fluid compartment, i. e. with the pressure difference between the pressure p.sub.2 within the filtrate compartment and the pressure p.sub.3 within the backwashing compartment. The process of filtration at the remaining filter surface of the pressure filter is not influenced by the backwashing.
According to the known pressure filter the backwashing compartment includes at least one suction chamber open in a slot-like manner towards the filter surface. This suction chamber is formed as longitudinal chamber extending from a central drive shaft for the backwashing head radially outwardly, which is also true for the slot-like aperture. Accordingly, when the backwashing head rotates the slot-like aperture sweeps over the filter surface in the form of a radial beam emanating from the center of the pressure filter. Obviously, when this is the case the apertures disposed at different radial points of the filter surface are exposed to the slot-like aperture or are subjected to the corresponding suction process over different periods of time since, upon rotation of the backwashing head, different radial points of the slot-like aperture move over different distances within the same unit. This has the result that the whole filter surface is unevenly cleaned by the backwashing so that the efficiency of the filter suffers hereby, the filter surface wears early and/or additional still-stand times of the filter occur or additional measures of maintenance have to be carried out.